OK, so I managed to add the sites with an online converter from OS to Lat/Long, but I'm not sure if the results are correct. Also, unlike with the OS co-ords, the Lat/Long entries don't give nearest town and supplementary info. Again, any ideas?
Ha ha, cracked it. After converting to Lat/Long and adding the site using that, then go back and edit the details and add the OS co-ords and Bob's your uncle! Sorry for the hassle, but it's worth keeping the topic on file methinks.